An upper limit for the water outgassing rate of the main

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An upper limit for the water outgassing rate of the main
Astronomy
&
Astrophysics
A&A 546, L4 (2012)
DOI: 10.1051/0004-6361/201220169
c ESO 2012
Letter to the Editor
An upper limit for the water outgassing rate of the main-belt
comet 176P/LINEAR observed with Herschel /HIFI
M. de Val-Borro1, , L. Rezac1 , P. Hartogh1 , N. Biver2 , D. Bockelée-Morvan2 , J. Crovisier2 , M. Küppers3 , D. C. Lis4 ,
S. Szutowicz5 , G. A. Blake4 , M. Emprechtinger4 , C. Jarchow1 , E. Jehin6 , M. Kidger7 , L.-M. Lara8 , E. Lellouch2 ,
R. Moreno2 , and M. Rengel1
1
2
3
4
5
6
7
8
Max Planck Institute for Solar System Research, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany
LESIA, Observatoire de Paris, CNRS, UPMC, Université Paris-Diderot, 5 place Jules Janssen, 92195 Meudon, France
Rosetta Science Operations Centre, ESAC, European Space Agency, 28691 Villanueva de la Cañada, Madrid, Spain
California Institute of Technology, Pasadena, CA 91125, USA
Space Research Centre, Polish Academy of Sciences, Warsaw, Poland
Institute d’Astrophysique et de Geophysique, Université de Liège, Belgium
Herschel Science Centre, ESAC, European Space Agency, 28691 Villanueva de la Cañada, Madrid, Spain
Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
Received 5 August 2012 / Accepted 25 August 2012
ABSTRACT
176P/LINEAR is a member of the new cometary class known as main-belt comets (MBCs). It displayed cometary activity shortly
during its 2005 perihelion passage, which may be driven by the sublimation of subsurface ices. We have therefore searched for
emission of the H2 O 110 –101 ground state rotational line at 557 GHz toward 176P/LINEAR with the Heterodyne Instrument for the
Far Infrared (HIFI) onboard the Herschel Space Observatory on UT 8.78 August 2011, about 40 days after its most recent perihelion
passage, when the object was at a heliocentric distance of 2.58 AU. No H2 O line emission was detected in our observations, from
which we derive sensitive 3-σ upper limits for the water production rate and column density of <4×1025 mol s−1 and of <3×1010 cm−2 ,
respectively. From the peak brightness measured during the object’s active period in 2005, this upper limit is lower than predicted
by the relation between production rates and visual magnitudes observed for a sample of comets at this heliocentric distance. Thus,
176P/LINEAR was most likely less active at the time of our observation than during its previous perihelion passage. The retrieved
upper limit is lower than most values derived for the H2 O production rate from the spectroscopic search for CN emission in MBCs.
Key words. comets: individual: 176P/LINEAR – submillimeter: planetary systems – techniques: spectroscopic
1. Introduction
Classical main-belt asteroids are small bodies that orbit the Sun
in low inclination and low eccentricity orbits between the orbits of Mars and Jupiter. Physically, asteroids are thought to be
devoid of volatiles, while comets are icy bodies that become
active in the inner solar system thanks to the sublimation of
ices, mostly water. Comets originate in the outskirts of the solar system beyond the snow line, where temperatures in the solar
nebula were low enough for water to condense onto icy grains
(Hayashi 1981). A new class of bodies has been discovered recently, the so-called main-belt comets (MBCs), which have orbital properties that are indistinguishable from standard asteroids
with a Tisserand parameter with respect to Jupiter that is greater
than three, and they display cometary activity in the form of a
dust tail during part of their orbit. Numerical simulations have
shown that these objects are not comets from the Kuiper belt or
Oort cloud that have been recently transferred to orbits within
the main belt, but instead are most likely formed in situ at their
current locations (Fernández et al. 2002).
Herschel is an ESA space observatory with science instruments
provided by European-led Principal Investigator consortia and with important participation from NASA.
Current address: department of Astrophysical Sciences, Princeton
University, NJ 08544, USA. e-mail: [email protected]
Theoretical models suggest that the snow line was initially
close to the Mars orbit due to the absorption of stellar radiation
by dust (Sasselov & Lecar 2000; Lecar et al. 2006). Therefore,
objects formed at their current locations in the outer asteroid belt
may have been able to accumulate water ice in subsurface reservoirs, despite the effect of solar radiation. Determining the composition of this class of objects can provide important clues to
both the thermal properties that allow water to survive in subsurface layers and the distribution of volatile materials in the solar
nebula to constrain planet formation mechanisms. Additionally,
MBCs may have played an important role in the delivery of water and other volatiles to the inner solar system, including the
Earth.
The MBC 176P/LINEAR (hereafter 176P) was discovered
in 1999 and originally categorized as asteroid 118401 LINEAR.
This object belongs to the Themis asteroid family. Cometary
activity was reported for this object around perihelion in 2005
(Hsieh et al. 2011) by the Hawaii Trails project (HTP; Hsieh
& Jewitt 2006; Hsieh 2009). It displayed a mean photometric
excess of ∼30% during a month-long active period around its
perihelion passage, consistent with an approximate total dust
mass-loss of ∼7 × 104 kg (Hsieh et al. 2011). Although ice sublimation is expected to trigger MBC activity, gas emission has
never been directly detected in these objects owing to their low
activity, which requires very sensitive observations. Herschel
proves to be the most sensitive instrument for directly observ-
Article published by EDP Sciences
L4, page 1 of 4
A&A 546, L4 (2012)
ing water in a distant comet (e.g. Bockelée-Morvan et al. 2010).
In this paper we present the Herschel observation of the 110 –101
fundamental rotational transition of H2 O at 557 GHz in 176P.
This observation is intended to test the prediction that the observed cometary activity of MBCs is driven by sublimation of
water ices and to constrain the production process.
2. Observations
The MBC 176P was observed with the Heterodyne Instrument
for the Far Infrared (HIFI; de Graauw et al. 2010), one of the
three instruments onboard the ESA Herschel Space Observatory
(Pilbratt et al. 2010), within the framework of the Herschel
guaranteed-time key program “Water and related chemistry in
the solar system” (Hartogh et al. 2009). HIFI provides very
high-resolution spectroscopy that can resolve the line shape
and enable the determination of accurate production rates (e.g.,
Hartogh et al. 2010). 176P was the best MBC target in terms of
its visibility close to the perihelion passage and anticipated line
strength to be observed by Herschel during the mission lifetime.
It passed its perihelion on 30 June 2011 at a distance of 2.57 AU
from the Sun and was observed by Herschel on UT 8.78 August
2011 with a total on-target integration time of 4.8 h, when it was
at a heliocentric distance of 2.58 AU and a distance of 2.55 AU
from the satellite (Herschel ObsID 1342225905). The object was
tracked using an up-to-date ephemeris provided by the JPL’s
Horizons system.
The line emission from the fundamental (110 –101 ) rotational
transition of ortho-water at 557 GHz was searched for in the upper sideband of the HIFI band 1a mixer. The observation was
performed in the frequency-switching observing mode with a
frequency throw of 94.5 MHz, using both the wide band spectrometer (WBS) and the high-resolution spectrometer (HRS).
In this observing mode there is no need to observe a reference position on the sky and the on-target integration time is
maximized. However, the statistical noise may be underestimated for observations in frequency-switched mode owing to
uncertainties in baseline removal (Bockelée-Morvan et al. 2012).
The spectral resolution of the WBS is 1 MHz (∼0.54 km s−1
at the frequency of the observed line), while the HRS was
used in its high-resolution mode with a resolution of 120 kHz
(∼0.065 km s−1 ). The main beam brightness temperature scale
was computed using a beam efficiency of 0.75 and a forward
efficiency of 0.96. The folded spectrum was obtained by averaging the original spectrum with a shifted and inverted copy.
Horizontal and vertical polarizations were averaged, weighted
by the root mean square amplitude, to increase the signal-tonoise ratio. The pointing offset of horizontal and vertical polarization spectra is 6. 6 in band 1a, corresponding to approximately
20% of the half-power beam width at the observed frequency.
3. Data analysis
The data analysis was performed using the Herschel interactive
processing environment (HIPE) software package (Ott 2010).
The standard HIFI processing pipeline v7.1.0 was used to reduce the data to calibrated level-2 data products. The frequencyswitching observing mode introduces a strong baseline ripple.
To obtain a reliable estimate of the noise present in the measured data, the baseline has to be removed, which is usually accomplished by fitting a linear combination of sine waves using
the Lomb-Scargle periodogram technique. Nevertheless, the instrumental processes responsible for the baseline are in general
L4, page 2 of 4
combinations of linear distortions of different components in the
receiver/spectrometer subsystems with a small fraction of nonlinear processes, which may cause an aperiodicity in the ripple.
Analyzing such contaminated signals by assuming a linear relation among the signal components (a fundamental assumption
for all the Fourier-based techniques) is not always suitable, depending on the degree of nonlinearity. In this work, we utilize a
relatively novel approach specifically developed for analysis of
aperiodic and nonlinear signals – the Hilbert-Huang Transform
(HHT; Huang et al. 1998, 1999; Battista et al. 2007). This approach combines the empirical mode decomposition (EMD) procedure, which decomposes the original signal into its intrinsic
mode functions (IMFs; representing the different modes of oscillations) with the Hilbert transform that can be then used in computing the instantaneous frequencies. The EMD technique extracts all the oscillatory modes, including all the baseline ripple
components (the smooth low-frequency modes), as well as the
highest frequency components, usually the noise. Another important property of IMFs is that they obey a simple additive rule
to reconstruct the original signal exactly. This makes the EMD
approach an accurate and versatile tool that has been successfully used in many areas from atmospheric science to cosmology such as denoising and detrending, as well as the time series
analysis tool for identifying periodic and quasi-periodic features
(see Duffy 2004; Battista et al. 2007, and references therein).
In this work we applied the EMD technique to the measured
WBS and HRS spectra to obtain the highest frequency IMFs
(dominated by Gaussian noise), which are usually the first modes
(Duffy 2004). Some of the modes obtained from the decomposition of the HRS spectrum are shown in the lower panels of Fig. 1.
Then, the root mean square (rms) noise of the brightness temperature in each spectrum is used to derive a 3-σ upper limit for the
H2 O production rate shown in Table 1. The brightness temperature rms of the WBS and HRS spectra differ owing to their different spectral resolutions as predicted by the Herschel observation
planning tool (Hspot). The rms agrees with the value derived
using the Lomb-Scargle periodogram method to determine the
frequencies of the baseline. Typically the Lomb-Scargle implementation requires 20 to 30 sinusoidal components to achieve a
good fit of the baseline ripple, while the HHT analysis provides
a good estimate of the noise with eight modes and has an additional advantage in computing speed. Fitting a narrow frequency
range of ∼40 MHz around the water line with a high-order polynomial tends to underestimate the noise level determined from
the whole band by about a factor of two. However, the difference introduced by the baseline fitting method is smaller than
the uncertainty in the upper limit for the outgassing rate due
to unknown model parameters, and therefore our data reduction
method does not modify the conclusions. The EMD and HHT
reduction methods applied to the baseline removal and denoising of the Herschel/HIFI data will be described in detail in a
future work (Rezac et al., in prep.). We show the baseline subtracted HRS spectrum in Fig. 2 with the expected line emission
overplotted.
4. Outgassing rate
There is no evidence of H2 O emission in our observation, although it is expected that the object’s dust emission activity
is driven by the sublimation of subsurface material as it approaches perihelion. A molecular excitation model based on
the publicly available accelerated Monte Carlo radiative transfer code ratran (Hogerheijde & van der Tak 2000) is used to
calculate the population of the rotational levels of water as a
M. de Val-Borro et al.: An upper limit for the water outgassing rate of 176P/LINEAR
v [km s−1 ]
60
40
20
0
−20
−40
3.8
3.75
20
T mB [mK]
T mB [mK] T mB [mK]
T mB [mK]
T mB [K]
3.85
IMF 8
0
−20
8
4
0
−4
−8
4
2
0
−2
−4
10
IMF 7
IMF 6
IMF 1
0
−10
556.85
556.9
556.95
f [GHz]
557
Fig. 1. Original HRS subband 1 spectrum of the H2 O 110 –101 line at
556.936 GHz observed on UT 8.78 August (upper panel), and several
low and high frequency components of the spectrum determined using
the EMD analysis (four lower panels) with labels indicating the mode
number. The vertical axis is the calibrated main beam brightness temperature. The lower horizontal axis is the upper sideband frequency,
while the upper axis shows the velocity with respect to the nucleus’s
rest frame.
Table 1. Standard deviation of the brightness temperature and line
area, and retrieved 3-σ upper limits of the H2 O production rate in
176P/LINEAR.
Spectrometer
WBS
HRS
σT mB
(K)
σ T mB dv
(K km s−1 )
Q H2 O a
(mol s−1 )
6.598 × 10−4
1.998 × 10−3
6.172 × 10−4
6.365 × 10−4
<2.08 × 1025
<2.14 × 1025
Notes. (a) Production rates derived for a gas kinetic temperature of 20 K,
expansion velocity of 0.5 km s−1 , and an electron density scaling factor
of xne = 0.2.
function of the nucleocentric distance. The code includes collisional effects and infrared fluorescence by solar radiation to
derive the production rates. We used the one-dimensional spherically symmetric version of the code following the description
outlined in Bensch & Bergin (2004) that has been used to analyze Herschel and ground-based cometary observations (see e.g.
Hartogh et al. 2010, 2011; de Val-Borro et al. 2010, 2012). The
model input parameters are the gas kinetic temperature, which
controls the molecular excitation in the collisional region, and
the electron density. We assume a gas kinetic temperature in
the range 20–40 K. The electron density and temperature profiles from Biver (1997) are adopted. Since the electron density
in the coma is not well constrained, an electron density scaling factor of xne = 0.2 with respect to the standard profile derived from observations of comet 1P/Halley has been used (e.g.
Hartogh et al. 2010). The expansion velocity is assumed to be
Fig. 2. High-frequency component of the HRS spectrum of the
H2 O 110 –101 line at 556.936 GHz observed on UT 8.78 August with
overplotted synthetic spectrum of the 3-σ upper limit shown as the
dashed line. The vertical axis is the calibrated main beam brightness
temperature. The lower horizontal axis is the upper sideband frequency,
while the upper axis shows the velocity with respect to the nucleus’s
rest frame.
constant in the coma, and the radial gas density profile for water was obtained using the standard spherically symmetric Haser
distribution. The gas expansion velocity derived from the preand post-perihelion evolution of comet C/1995 O1 (Hale-Bopp)
is given by vexp = 1.12 × rh−0.41 km s−1 (Biver et al. 1997),
which corresponds to 0.76 km s−1 at 176P’s heliocentric distance. Since Hale-Bopp was a very active comet, this expansion
velocity is a very conservative upper limit. A thermal velocity
of 0.35 km s−1 is obtained from the temperature expected at the
subsolar point where ice sublimates. Then, as cometary atmospheres are formed by quasi-adiabatic expansion, the gas accelerates as it expands, as observed with the large field of view of
the 18-cm OH observations. For low-activity and distant comets
an expansion velocity close to 0.5 km s−1 is determined from the
shapes of the OH line observed with the Nançay radio telescope
(Tseng et al. 2007), but observations for QOH < 1028 mol s−1 are
lacking, and observations at rh > 2 AU are rare. Odin observations of the H2 O 557 GHz line toward the active comet C/2003
K4 (LINEAR) at 2.2 AU from the Sun are consistent with an expansion velocity on the order of 0.5 km s−1 (Biver et al. 2007).
The dust particles ejected by 176P have an approximate velocity
or ∼5 m s−1 calculated from numerical simulations to match optical observations (Hsieh et al. 2011). However, dust is expected
to be much slower than gas if it is formed by large and heavy
particles with very low outgassing rates.
The mean of the derived upper limits on the total H2 O column density integrated within the beam for the considered model
parameters is NH2 O < 3 × 1010 cm−2 . For a low expansion velocity characteristic of weak comets, we derive a sensitive 3-σ
upper limit on the water production rate of <2.1 × 1025 mol s−1
from the WBS and HRS data (see Table 1). An upper limit of
<4×1025 mol s−1 is derived from the mean of the WBS and HRS
upper limits for gas expansion velocities between 0.4–0.7 km s−1
and gas kinetic temperatures between 20–40 K.
L4, page 3 of 4
A&A 546, L4 (2012)
With the exception of 133P/Elst-Pizarro, our upper limit for
the H2 O production rate is more stringent than any of those derived in other MBCs from the spectroscopic search for CN emission in the optical when the objects were active. In addition, it
does not require the uncertain assumption of a QCN /QH2 O value.
Adopting a QCN /QH2 O mixing ratio of 10−3 the upper limits are
QH2 O < 1.3 × 1024 mol s−1 in 133P/Elst-Pizarro (Licandro et al.
2011), QH2 O < 1.4 ×1026 mol s−1 in P/2008 R1 (Garradd) (Jewitt
et al. 2009), QH2 O < 1.3 × 1027 mol s−1 in P/2006 VW139 (Hsieh
et al. 2012b), QH2 O < 6 × 1026 mol s−1 in P/2010 R2 (La Sagra)
(Hsieh et al. 2012c), and QH2 O < 9 × 1026 mol s−1 in the collisionally disrupted main belt object (596) Scheila (Hsieh et al.
2012a). However, no direct search for H2 O has been carried out
before, and there are substantial uncertainties in the estimation
of the water production from the CN emission given the wide
range of observed QCN /QH2 O ratios in comets and their dependence on heliocentric distance. Therefore, these previously published values can only be considered as order-of-magnitude approximations.
5. Discussion
We observed water emission in 176P with Herschel/HIFI to test
the prediction that cometary activity in MBCs is driven by sublimation of water ices from the nucleus. There are several mechanisms that have been proposed to drive mass loss from small
bodies, including sublimation of subsurface ices, rotational instability, impact ejection and thermal fracture (see Bertini 2011;
Jewitt 2012, for recent reviews of MBC physical properties and
activation mechanisms). The cometary activity observed in 176P
was initially found to suggest the presence of sublimating subsurface ice that may have been exposed by recent collisions
(Hsieh et al. 2011). This view is supported by the detection of
water ice absorption in spectroscopic observations centered at
3.1 μm of the surface of the largest asteroid of the Themis asteroid family, 24 Themis, which belongs to the same dynamical
family as 176P (Rivkin & Emery 2010; Campins et al. 2010),
although it has been claimed that the measured spectra are consistent with the transmission spectra of goethite (Beck et al.
2011). From the search for the H2 O 110 –101 rotational line at
557 GHz in 176P, a 3-σ upper limit for the H2 O production rate
of <4 × 1025 mol s−1 is derived from the WBS and HRS data, for
gas expansion velocities between 0.4–0.7 km s−1 and gas kinetic
temperatures between 20 and 40 K. Using the peak value of the
R-band magnitude, m(1, 1, 0) = 15.35 ± 0.05, measured when
the object was active in late 2005 at a heliocentric distance of
2.58 AU, a V-band magnitude of mV (1, rh , 0) = 17.8 is obtained
(Hsieh et al. 2009). Since the cometary activity in 176P is indicative of ice sublimation with a 30% contribution of the coma to
the total brightness, the scaling relation between gas production
rates and heliocentric magnitudes from Jorda et al. (2008) predicts a water production rate of approximately 1.0×1026 mol s−1 .
This correlation has been obtained for a sample of 37 comets
with heliocentric distances between 0.32–4.53 AU. Thus, if ice
sublimation is the driving mechanism of 176P’s activity, the derived H2 O production rate is too low by about a factor of two to
explain the activity level during its 2005 perihelion passage. It is
unlikely that sublimation of carbon monoxide ice is the source of
the activity in MBCs since the temperature in the region where
the comet formed could not have allowed condensation of CO.
We note that the Jorda et al. (2008) correlation should be taken
with some care because none of the comet measurements were
obtained at the brightness level of 176P. From the dust production rate of 0.07 kg s−1 estimated by Hsieh et al. (2011), a water
L4, page 4 of 4
production rate of 2.3 × 1024 mol s−1 is inferred assuming a dustto-gas ratio of one. However, there are significant uncertainties
in the value of the dust production rate determined from photometric data, and additionally the dependence on the dust-togas ratio with heliocentric distance is poorly constrained. Midinfrared photometric observations of 176P on 23–24 April 2010
by the Wide-field Infrared Survey Explorer (WISE) mission did
not provide any indication of a coma (Bauer et al. 2012), and
there are no other published infrared or optical data closer to the
last perihelion passage. We conclude that water was not detected
in our observation because the water production rate was lower
than 4 × 1025 mol s−1 or the object was not active during our observation, so a more detailed study is needed to shed light on the
activation mechanism in MBCs.
Acknowledgements. HIFI has been designed and built by a consortium of institutes and university departments from across Europe, Canada, and the United
States under the leadership of SRON, Netherlands Institute for Space Research,
Groningen, The Netherlands, and with major contributions from Germany,
France, and the US. HIPE is a joint development by the Herschel Science Ground
Segment Consortium, consisting of ESA, the NASA Herschel Science Center,
and the HIFI, PACS, and SPIRE consortia. Support for this work was provided
by NASA through an award issued by JPL/Caltech. M.d.V.B. was supported by
the Special Priority Program 1488 of the German Science Foundation. S.S. was
supported by Polish MNiSW funds (181/N-HSO/2008/0). We acknowledge the
referee, H. Campins, for his comments.
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